1. Field of the Invention
The present invention relates to an exposure method, an exposure apparatus and a device producing method, for example, to an exposure method, an exposure apparatus which are used in a lithography process for producing devices such as semiconductor devices, image pickup devices (CCDs etc.), liquid crystal display devices, plasma display devices, thin film magnetic heads, and to a device producing method using the exposure method. The invention is preferably used for controlling exposure amount using a scan exposure type projection exposure apparatus employing a step and scan system using an exposure beam from a pulse light source.
2. Description of the Related Art
Conventionally, as one of basic functions of the projection exposure apparatus used when semiconductor devices or the like are produced, there exists an exposure amount control function for keeping, within an appropriate range, an integrated exposure amount of each point in each shot region of a wafer (or glass plate or the like) as a substrate to be exposed on which photoresist (photosensitive material) is applied. As an exposure amount control in a conventional full field exposure apparatus such as a stepper, even if either of a continuous light source such as a mercury lamp and a pulse laser light source such as an excimer laser light source is used, a cut off control is basically carried out. In the cut off control, exposure is continued until an integrated value of exposure amount on a wafer which is indirectly measured through an integrator sensor comprising a photoelectric detector in an illumination optical system exceeds a predetermined critical level corresponding to a predetermined set exposure amount (target exposure amount). When the pulse laser light source is used as an exposure light source, since each pulse laser light has a variation in its energy, exposure is carried out using a plurality of pulse laser light having a predetermined number of minimum exposure pulses or greater, thereby obtaining a desired control and reproduction precision of exposure amount.
Further, in the scan exposure type projection exposure apparatus such as the step and scan type apparatus used in recent years, a system (open exposure amount control system) for controlling the exposure amount by simply integrating the light amount of each pulse of light as exposure light (exposure, beam) is used. Whereas, as disclosed in U.S. Pat. No. 5,627,627, there is proposed a system (pulse-by-pulse exposure amount control system) for controlling the energy of each pulse of light by measuring the integrated exposure amount with respect to a region included in a slit-shaped exposure region (illumination field) in a scanning direction on a wafer in a pulse-by-pulse manner in real time, and by individually calculating target energy of the next pulse of light based on the integrated exposure amount. In this pulse-by-pulse exposure amount control system, it is possible to reduce the variation in the integrated exposure amount as compared with the open exposure amount control system, and the pulse-by-pulse exposure amount control system can also be applied to the full field exposure type projection exposure apparatus.
By controlling the exposure amount with high precision in this manner, the size precision of pattern (resist pattern) of photosensitive material after development is enhanced, resolution that is more closer to threshold resolution determined in accordance with wavelength (exposure wavelength) of the exposure light, a numerical aperture of the projection optical system and the like can be obtained. The exposure wavelength itself has been further shortened in order to further enhance the threshold resolution, and in recent years, a tendency of using an Arf excimer laser (wavelength of 193 nm) as an exposure light source of the next generation of the KrF excimer laser (wavelength of 248 nm) is increasing.
Under the present circumstances, an optical material having a practical transmittance that can be used as glass material for a refraction member (such as lens) constituting the projection optical system is quartz glass (synthetic quartz) and fluorite. Since the fluorite is expensive, it is possible to reduce the producing cost of the projection optical system by using the quartz glass as much as possible.
However, since the excimer laser light is pulsed light, and if the optical material such as quartz glass is irradiated with the pulsed light of high energy in vacuum ultraviolet region, it is known that its transmittance is varied with time constant on the order of some tens seconds. This phenomenon is reversible to some degree, and if the radiation of pulsed light is stopped, the transmittance of the optical material is restored toward a direction of its initial value with a predetermined time constant. Therefore, when all shot regions on the wafer are sequentially exposed using the pulsed light, the transmittance of the projection optical system is gradually varied.
If the transmittance of the projection optical system is varied in this manner, the relation between the light amount of the exposure light detected by the above-mentioned integrator sensor and the light amount on an image plane of the projection optical system is varied. Therefore, if the exposure amount is controlled based on the relation measured before exposure (correlation between the integrator sensor and the image plane) and the detection result of the integrator sensor, an error is generated in the integrated exposure amount. For this reason, in the projection optical system using the ArF excimer laser light to obtain higher resolution, there is a problem that the target resolution can not be obtained.
Similarly, also when the transmittance variation is generated in an optical member from the integrator sensor to a reticle as a mask in the illumination optical system, the control precision of the exposure amount is lowered.
In this regard, it is known that the variation amount of the transmittance in the short term can be reduced by controlling the doped amount of hydrogen ion (H+) in components included in the quartz glass. However, in such a quartz glass, deterioration (compaction) of characteristics due to irradiation of pulsed light over long term is increased. For this reason, although it is possible to use hydrogen ion amount controlled quartz glass where the quartz glass can easily be replaced as in the illumination optical system, but it is preferable not to use the hydrogen ion amount controlled quartz glass where it is difficult to replace the quartz glass, as in the projection optical system. As a result, since there is a tendency that quartz glass whose transmittance is varied in the short term is used especially in the projection optical system, it is desired to develop an exposure method and an exposure apparatus capable of obtaining high control precision of exposure amount even if the transmittance is varied.
When a reflectivity of a reflection member in the projection optical system comprising a catadioptric system for example is also varied in a short term in addition to the transmittance, it is necessary to appropriately control the exposure amount to cope with it.
In view of the above circumstances, it is a first object of the present invention to provide an exposure method capable of obtaining high control precision of exposure amount on an object to be exposed (such as wafer) even when an optical member whose transmittance or reflectivity is gradually varied is used in an optical system.
It is a second object of the present invention to provide an exposure method capable of obtaining high control precision of exposure amount on an object to be exposed even if the transmittance (the entire, transmittance including the reflectivity) of the optical system when the object is exposed with pulsed light.
Further, it is another object of the invention to provide an exposure apparatus capable of using the above exposure method and a producing method of a device using the above exposure method.
According to a first aspect of the present invention, there is provided an exposure method which illuminates a first object with an exposure beam and exposes a second object through a projection system with the exposure beam passing through a pattern of the first object, comprising:
with a correlation between a light amount of the exposure beam detected at a measuring point on an optical path to the first object and a light amount of the exposure beam on an image plane of the projection system being previously obtained,
when an integrated exposure amount with respect to the second object is controlled based on the light amount of the exposure beam detected at the measuring point and the correlation at the time of exposure of the second object, forecasting a variation amount of transmittance of the projection system in accordance with a process condition and exposure progression, and renewing the correlation based on the forecast variation amount of the transmittance.
According to the exposure method of the first aspect, of the invention, a coefficient for obtaining the light amount on the image plane from the light amount at the measuring point for example is previously obtained as the correlation, and the variation ratio of the transmittance of the projection system is previously obtained as a function of the process condition and the exposure progression. Then, the variation amount of the transmittance is obtained during exposure based on the function, and the coefficient can be corrected by multiplying the coefficient by the transmittance after it was varied. By multiplying the measured value of the light amount at the measuring point by the corrected coefficient, the light amount of the exposure beam on the second object can be indirectly obtained, and the high exposure amount control precision can be obtained by controlling the exposure amount such that the integrated, exposure amount on the second object obtained by integrating the light amount becomes the target exposure amount (set exposure amount).
In this case, by correcting the coefficient using the average value of the spacial transmittance variation of the projection system, the error in the exposure amount in each sectioned region (shot region) on the second object can be divided with respect to the average value with small calculation amount and minimized.
Although the projection system is used as the optical system for forecasting the variation amount of the transmittance in the exposure method of the first aspect of the present invention, when an illumination system in which the first object is irradiated with the exposure beam is provided and the light amount of the exposure beam is detected at a predetermined measuring point in the illumination system, an optical member of a portion of the illumination system disposed between the measuring point and the first object and an optical system including the projection system are used as an optical system for forecasting the variation amount of the transmittance. That is, an optical member disposed on an optical path from the measuring point to the second object in the illumination system is the optical system for forecasting the variation amount of the transmittance.
As one example, a wavelength of the exposure beam is 200 to 170 nm, and the projection system includes a refraction member comprising quartz glass. The quartz glass has a tendency that its transmittance with respect to light (especially pulsed light of high output in vacuum ultraviolet region is varied in the short term, and the transmittance variation can mathematically be handled substantially correctly as a general linear system by cutting (variation) of Sixe2x80x94X bonding and recombination (relaxation) thereof. By forecasting and calculating the variation amount of the transmittance caused during the exposure in which the exposure beam is irradiated and the relaxation amount of the transmittance caused during step moving or replacing of the second object, the exposure amount can be controlled with high precision based on the forecast values. Therefore, the control method of forecasting the variation amount of the transmittance as in the present invention is especially effective.
As one example, the exposure beam is pulsed light, and the process condition is at least one of an illumination condition of the first object, a kind of the pattern on the first object, size (chip size) of the illumination region, on the first object, pulse energy density (fluence) of the exposure beam, and cumulative using period of material of the optical member constituting the projection system. The exposure progression is expressed by at least one item of data of emitting time of the exposure beam and non-emitting time of the exposure beam.
For example, if the pattern density is locally varied depending upon a kind of pattern on the first object, the transmittance distribution is varied due to variation in intensity distribution of the exposure beam passing through the projection system, and the uneven illumination is varied in the exposure region on the second object. In this case also, it is possible to obtain appropriate exposure amount in average by forecasting the unevenness in transmittance distribution in accordance with the kind of the pattern.
It is preferable to use a variation amount of the transmittance with respect to the illumination power of the exposure beam and a time constant of variation of the transmittance as control parameters for forecasting the variation amount of the transmittance of the projection system, and to independently hold the control parameters for each of the illumination conditions of the first object. If the control parameters (control constants) are independently held for each of the illumination conditions, it is possible to excellently correct the exposure amount under any illumination condition.
Next, according to a second aspect of the invention, in the exposure method of the first aspect of the invention, there is provided a second exposure method wherein when the non-emitting time of the exposure beam exceeds a predetermined time, the transmittance of the projection system is measured, the forecast value of the variation amount of the transmittance of the projection system is reset.
According to the exposure method of the second aspect of the present invention, when there exists a non-emitting time exists as compared with the stepwisely moving time of the second object, such as the time of replacement, alignment or the like of the second object, there is an adverse possibility that not only the relaxation caused by recombination of the above-described cut bonding, but also a physical phenomenon which is difficult to be forecast such as the adherence of a contaminant remaining in a space of optical path and in a chamber of the exposure apparatus to the optical system may bring about a variation of the transmittance which is not negligible and therefore, the precision of the forecasting of the transmittance may be deteriorated in some cases. When the forecasting precision of the transmittance is deteriorated, if the transmittance of the projection system is measured in real time for verification and the forecast value based on the forecasting control equation is reset to the initial value, the control precision of the exposure amount is enhanced.
If the transmittance of the projection system is measured after a long non-illumination time and measured result showing that the forecasting precision of the transmittance is deteriorated is obtained, dummy illumination after the non-illumination time may be carried out, and the contaminant adhered to the optical system may be cleaned with light.
Similarly, when exposure is continuously carried out under different illumination conditions such as a so-called double exposure in which each sectioned region (shot region) on the second object is exposed using two kinds of mask patterns under two kinds of illumination conditions, the forecasting precision is also deteriorated when an relaxation phenomenon caused by the immediately preceding different illumination condition and a variation phenomenon caused by an illumination condition from now on are mixed. In such a case, when it is judged that the deterioration of the forecasting precision exceeds a predetermined level, if the exposure is stopped to measure the transmittance in real time for verification and the forecasting control equation is reset, the control precision of the exposure amount is enhanced.
Further, when it is forecast that the control parameters (control constants) themselves will also be varied gradually by long-term illumination of exposure beam, it is preferable to make the parameters renewable constants which are to be renewed by periodically measuring the control parameters. With this method, it is possible to excellently correct the exposure amount for a long time.
It is preferable that whenever a predetermined time is elapsed, the transmittance distribution with respect to the exposure beam of the projection system is measured, and if the variation of the transmittance distribution exceeds a predetermined tolerance, the illumination of the exposure beam is stopped to alleviate the variation in the transmittance of the projection system, or the first object is removed to irradiate the entire illumination region with the exposure beam to alleviate the variation. If the variation in the transmittance distribution becomes excessively great, there is an adverse possibility that the uneven exposure amount on the second object may exceed the tolerance. In such a case, if the transmittance is uniformed, the exposure amount distribution is uniformed.
Next, according to a third aspect of the present invention, there is provided an exposure apparatus, comprising:
an exposure light source which generates an exposure beam;
an illumination system which irradiates a first object with the exposure beam;
a projection system which irradiates a second object with the exposure beam passing through a pattern of the first object;
a first detector which detects a light amount of the exposure beam at a measuring point in the illumination system;
a second detector which detects a light amount of the exposure beam on an image plane of the projection system;
an exposure amount control system which stores a correlation between an output of the first detector and an output of the second detector and controls an integrated exposure amount with respect to the second object based on the output of the first detector and the correlation; and
a forecasting control system which forecasts a variation amount of the transmittance of the projection system in accordance with a process condition and an exposure progression and renews the correlation based on the forecast variation amount of the transmittance.
Using such an exposure apparatus, the exposure method of each of the first and second aspects of the present invention can be used. Although the variation amount of the transmittance of the projection system is forecast in the third aspect of the present invention, it is also possible to forecast the variation amount of the transmittance of the optical system (including a portion of a plurality of optical members constituting the illumination system and the projection system) from the measuring point in thee integrator sensor to the second object. Since the most of variation amount of the transmittance is ascribable to the variation amount of the projection system, the variation amount of the transmittance of the optical system from the measuring point to the second object can substantially be considered as the variation amount of the transmittance of the projection system. In this case also, when the variation amount of the transmittance of the optical member (a portion of the illumination system) disposed between the measuring point and the projection system is great, this variation amount is taken into consideration of course.
In this case, it is preferable that an illumination condition switching member which switches illumination conditions with respect to the first object is provided in the illumination system, and the forecasting control systems calculates the forecast value of the variation amount of the transmittance of the projection system in accordance with the illumination condition.
It is preferable that a third detector, which measures light amount distribution of the exposure beam at the image plane of the projection system, is provided, and this third detector measures the transmittance distribution of the projection system.
Next, according to a fourth aspect of the present invention, there is provide a device producing method, comprising: transferring a mask pattern onto a work piece using the exposure method of the first or second aspect of the present invention. According the device producing method of the fourth aspect of the present invention, since if the exposure is carried out while using a pattern of the first object as a pattern, for the device and using the second object as the work piece (a substrate of the like such as a wafer), the integrated exposure amount can be controlled with high precision and therefore, it is possible to mass-produce a device, of high precision which is excellent in line width precision.